1. Field of the Invention
The present invention relates to semiconductor materials comprising tin and generally tellurium, and also at least one or two further dopants, and to thermoelectric generators and Peltier arrangements comprising said materials.
2. Description of the Background
Thermoelectric generators and Peltier arrangements as such have been known for some time. p- and n-doped semiconductors which are heated on one side and cooled on the other side transport electrical charges through an external circuit, and electrical work can be performed by a load in the circuit. The efficiency of conversion of heat to electrical energy achieved in this process is limited thermodynamically by the Carnot efficiency. Thus, at a temperature of 1000 K on the hot side and 400K on the “cold” side, an efficiency of (1000-400): 1000=60% would be possible. However, only efficiencies of up to 10% have been achieved to date.
On the other hand, when a direct current is applied to such an arrangement, heat is transported from one side to the other side. Such a Peltier arrangement works as a heat pump and is therefore suitable for cooling apparatus parts, vehicles or buildings. Heating via the Peltier principle is also more favorable than conventional heating, because more heat is always transported than corresponds to the energy equivalent supplied.
A good review of effects and materials is given, for example, by Cronin B. Vining, ITS Short Course on Thermoelectricity, Nov. 8, 1993, Yokohama, Japan.
At present, thermoelectric generators are used, for example, in space probes for generating direct currents, for cathodic corrosion protection of pipelines, for energy supply to light buoys and radio buoys and for operating radios and television sets. The advantages of thermoelectric generators lie in their extreme reliability. For instance, they work irrespective of atmospheric conditions such as atmospheric moisture; there is no fault-prone mass transfer, but rather only charge transfer. It is possible to use any fuels from hydrogen through natural gas, gasoline, kerosene, diesel fuel up to biologically obtained fuels such as rapeseed oil methyl ester.
Thermoelectric energy conversion thus fits extremely flexibly into future requirements such as hydrogen economy or energy generation from renewable energies.
A particularly attractive application is the use for converting (waste) heat to electrical energy in motor vehicles, heating systems or power plants. Thermal energy unutilized to date can even now by recovered at least partly by thermoelectric generators, but existing technologies achieve efficiencies of significantly below 10%, and so a large part of the energy is still lost unutilized. In the utilization of waste heat, there is therefore also a drive toward significantly higher efficiencies.
The conversion of solar energy directly to electrical energy would also be very attractive. Concentrators such as parabolic troughs can concentrate solar energy into thermoelectric generators, which generates electrical energy.
However, higher efficiencies are also needed for use as a heat pump.
Thermoelectrically active materials are rated essentially with reference to their efficiency. A characteristic of thermoelectric materials in this regard is what is known as the Z factor (figure of merit):
  Z  =                    S        2            ·      σ        κ  with the Seebeck coefficient S, the electrical conductivity σ and the thermal conductivity κ. Preference is given to thermoelectric materials which have a very low thermal conductivity, a very high electrical conductivity and a very large Seebeck coefficient, such that the figure of merit assumes a maximum value.
The product S2σ is referred to as the power factor and serves for comparison of the thermoelectric materials.
In addition, the dimensionless product Z·T is often also reported for comparative purposes. Thermoelectric materials known hitherto have maximum values of Z·T of about 1 at an optimal temperature. Beyond this optimal temperature, the values of Z·T are often significantly lower than 1.
A more precise analysis shows that the efficiency η is calculated from
      η    =                                        T            high                    -                      T            low                                    T          high                    ⁢                        M          -          1                          M          +                                    T              low                                      T              high                                            where      M    =                  [                  1          +                                    Z              2                        ⁢                          (                                                T                  high                                +                                  T                  low                                            )                                      ]                    1        2            (see also Mat. Sci. and Eng. B29 (1995) 228).
The aim is thus to provide a thermoelectric material having a maximum value of Z and a high realizable temperature differential. From the point of view of solid state physics, many problems have to be overcome here:
A high σ requires a high electron mobility in the material, i.e. electrons (or holes in p-conducting materials) must not be bound strongly to the atomic cores. Materials having high electrical conductivity σ usually simultaneously have a high thermal conductivity (Wiedemann-Franz law), which does not allow Z to be favorably influenced. Materials used at present, such as Bi2Te3, already constitute compromises. For instance, the electrical conductivity is lowered to a lesser extent by alloying than the thermal conductivity. Preference is therefore given to using alloys, for example (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 or Bi12Sb23Te65, as described in U.S. Pat. No. 5,448,109.
For thermoelectric materials having high efficiency, still further boundary conditions preferably have to be fulfilled. For instance, they have to be sufficiently thermally stable to be able to work under operating conditions over the course of years without significant loss of efficiency. This requires a phase which is thermally stable at high temperatures per se, a stable phase composition, and negligible diffusion of alloy constituents into the adjoining contact materials.
Doped lead tellurides for thermoelectric applications are described, for example, in WO 2007/104601. These are lead tellurides which, as well as a majority of lead, also comprise one or two further dopants. The particular proportion of the dopants, based on the formula (I) specified in the WO, is from 1 ppm to 0.05. Example 5 discloses Pb0.987Ge0.01Sn0.003Te1.001. This material actually includes the lowest lead content of the illustrative compounds. The materials thus have very high lead contents and only very low tin contents, if any.
WO 2007/104603 relates to lead germanium tellurides for thermoelectric applications.
These are ternary compounds of lead, germanium and tellurium, in which very high lead contents are again present.
For the production of a thermoelectric module, n- and p-conductors are always necessary. In order to arrive at a maximum efficiency of the module, i.e. at a maximum cooling performance in the case of a Peltier arrangement or a maximum generator performance in the case of a Seebeck arrangement, p-conductive and n-conductive material must be as well matched to one another as possible. This relates in particular to the parameters of Seebeck coefficient (ideally S(n)=−S(p)), electrical conductivity (ideally σ(n)=σ(p)), thermal conductivity (ideally λ(n)=λ(p)) and coefficient of thermal expansion (ideally α(n)=α(p)).
Proceeding from this prior art and the material requirements mentioned, it is an object of the present invention to provide thermoelectrically active materials which have a high thermoelectric efficiency and exhibit a suitable profile of properties for different application sectors. They should preferably include materials which, within the temperature range under application conditions (typically between ambient temperature and at least 150° C.), do not undergo any change in the mechanism of conduction.